Coatings having a high refractive index (n) are known from various applications, for example, optical lenses, antireflection coatings, planar waveguides or holographically writeable films. Coatings having high refractive indices can in principle be produced by various methods. By a purely physical route, metal oxides having a high refractive index, such as, for example, TiO2, Ta2O5, CeO2, Y2O3, are deposited in a high vacuum via plasma methods in the so-called “sputter process.” While refractive indices of more than 2.0 in the visible wavelength range can be achieved thereby without problems, the process is relatively complicated and expensive.
EP 0964019 A1 and WO 2004/009659 A1, for example, disclose organic polymers, for example, sulfur-containing polymers or halogenated acrylates (tetrabromophenyl acrylate, Polyscience Inc.), which inherently have a higher refractive index than conventional polymers and which can be applied to surfaces by simple methods from organic solutions according to conventional coating processes. However, the refractive indices are generally limited here to values up to about 1.7, measured in the visible wavelength range.
A further process variant which is becoming increasingly important is based on metal oxide nanoparticles, which are incorporated into organic or polymeric binder systems. The corresponding nanoparticle-polymer hybrid formulations can be applied to various substrates in a simple and economical manner, for example by means of spin coating. The achievable refractive indices are usually between the first-mentioned sputter surfaces and the layers of polymers having a high refractive index. With increasing nanoparticle contents, it is possible to achieve increasing refractive indices. For example, U.S. Pat. App. Pub. No. 2002/0176169 A1 discloses the preparation of nanoparticle-acrylate hybrid systems, the layers having a high refractive index containing a metal oxide, such as, for example, titanium oxide, indium oxide or tin oxide, and a UV-crosslinkable binder, for example based on acrylate, in an organic solvent. After spin coating, evaporation of the solvent and UV irradiation, corresponding coatings having a real part n of the refractive index of 1.60 to 1.95, measured in the visible wavelength range, can be obtained.
In addition to the physical properties, however, the processability and compatibility with other components are also important. Thus, organic materials which are obtained by photopolymerization, generally as homo- or copolymers of monomers having a high refractive index, play an important role, for example for the production of optical components, such as lenses, prisms and optical coatings (see, e.g., U.S. Pat. No. 5,916,987), or for the production of a contrast in holographic materials (see, e.g., U.S. Pat. No. 6,780,546). For such and similar applications, there is a need to be able to adjust the refractive index in a targeted manner and to vary it over ranges, for example by admixing components having a high refractive index.
For the abovementioned fields of use, polymers of olefinically unsaturated compounds, such as, preferably, (meth)acrylates, are typically used. In order to achieve a refractive index of 1.5 or higher, halogen-substituted aromatic (meth)acrylates or special alkyl methacrylates described in U.S. Pat. No. 6,794,471 can be used. Owing to their complicated preparation, the latter in particular are disadvantageous.
The suitability of substituted phenyl isocyanate-based urethane acrylates for the preparation of corresponding polymers has been described by Bowman (Polymer 2005, 46, 4735-4742).
The unpublished International Application PCT/EP2008/002464 discloses (meth)acrylates having a refractive index at λ=532 nm of at least 1.5, which are suitable for the production of optical data media, in particular those for holographic storage methods, and are based on industrially available raw materials. In this context, phenyl isocyanate-based compounds are also known, these always being based on unsubstituted phenyl rings on the isocyanate side.
In photopolymer formulations, acrylates having a high refractive index play a decisive role as a contrast-imparting component (U.S. Pat. No. 6,780,546). The interference field of signal and reference lightbeam (two planar waves in the simplest case) is mapped into a refractive index grating, which contains all information of the signal (the hologram), by the local photopolymerization at locations of high intensity in the interference field by the acrylates having a high refractive index. By illuminating the hologram only with the reference lightbeam, the signal can then be reconstructed. The strength of the signal thus reconstructed in relation to the strength of the incident reference light is referred to as diffraction efficiency, or DE below. In the simplest case of a hologram which is formed by the superposition of two planar waves, the DE is obtained from the quotient of the intensity of the light diffracted on reconstruction and the sum of the intensities of incident reference light and diffracted light. The higher the DE, the more efficient is a hologram with respect to the necessary quantity of reference light which is necessary in order to visualize the signal with a fixed brightness. Acrylates having a high refractive index are capable of producing refractive index gratings having a high amplitude between regions with the lowest refractive index and regions with the highest refractive index and thereby of permitting holograms having a high DE in photopolymer formulations